In this kind of silver-metal oxide type contact material produced by the internal oxidation process, the silver-cadmium oxide type material has been in wide use. Since the silver-cadmium oxide contact has relatively balancing properties of low contacting resistance it is used in relays, no-fuse breakers, breakers in the air and the like which operate in a wide currant range, from air and the like applicable to the current ranging from small to large. However, the use of cadmium composing the contact material is hazardous to workers' health during the production process. Instead, silver-tungsten, silver-tungsten carbide, silver-nickel and silver-graphite are used as a cadmium-free contact material, but the silver-tungsten and silver-tungsten carbide contacts are inferior to the silver-cadmium oxide due to the rise of temperature at the contacting point caused mainly by the increase of contacting resistance upon many time opening and closing operations in the air, and the silver-nickel and silver-graphite contacts are also inferior to the silver-cadmium oxide because of the welding resistance and arc erosion resistance when used in a range from medium to large current. Accordingly, as a contact of a switching unit in contact with the air, the actual usage ranges and operable conditions for these contact materials are considerably limited. Therefore, if an excellent contact material could be obtained without using cadmium, the advantages would be great.
As is known, this requirement is partly met by the silver-indium oxide-tin oxide contact material, but this material is, however, inferior to the silver-cadmium oxide type material in the welding and arc erosion resistances when used in a range from medium to large current.